How weather visibility is determined

Dear Tom,
Can you explain how visibility is determined?

John A. Wilson, Sycamore
Dear John,
Prior to automation, visibility was determined by weather observers who noted the most distant object visible at a known distance, such as a building, water tower or geographical landmark. After dark, the observer relied on unfocused lights as darkness made most daytime objects unusable. In recent years automated weather observing systems have been implemented and complex algorithms compute the visibility using data obtained by bouncing a laser beam back and forth between a transmitter and a receiver. Observers can still augment the computer visibility value during rapidly changing conditions or when the computer measurements seem inaccurate. Ten miles is the maximum visibility reported by the automated system, a value considered ample for aviation safety.